The typical implantable cardioverter/defibrillator (ICD) delivers an initial electrical countershock within ten to twenty seconds of arrhythmia onset, thereby saving countless lives. Improved devices have antitachycardia pacing capabilities in addition to cardioverting/defibrillating functions. These ICDs are capable of different initial responses to one or more tachycardia as well as a programmable sequence of responses to a particular arrhythmia.
The output energy level is generally set by a physician in accordance with a patient's capture threshold, determined at the time of heart implantation. This threshold represents the minimum pacing energy required to reliably stimulate a patient's heart. However, due to trauma associated with the stimulation, scar tissue grows at the interface between the implanted cardiac pacer leads and the myocardium. This scar tissue boosts the patient's capture threshold. To insure reliable cardiac capture, the output energy level is thus generally set at a level which is a minimum of two times greater than the initially measured capture threshold. A drawback to such an approach is that the higher stimulation level causes more trauma to the cardiac tissue than would a lower level of stimulation, and hence promotes the formation of scar tissue, thereby boosting the capture threshold.
The higher stimulation level also shortens battery life. This is not desirable, as a shorter battery life necessitates more frequent surgery to implant fresh batteries.
Another drawback is the potential for patient discomfort associated with this higher stimulation level. This is because the higher stimulation level can stimulate the phrenic or diaphragmatic plexus or cause intercostal muscle pacing.
Lastly, the higher stimulation is less effective, due to entry block.
A need therefore exists for an ICD that can achieve reliable cardiac capture with a lower stimulation level, thereby causing less damage to the heart, extending battery life, causing less pain to the patient and having greater therapeutic effectiveness than current ICDs. A need also exists for an ICD that can better entrain the heart and can entrain portions of the heart from a greater distance.